Excellent article and a good conversation thread. I also have the Neewer 4 pack of 4, 8, 16 and 32 PL/ND. Very nice design.. Color accuracy (as you can see from OP test is excellent). For those interested in color, assuming a totally neutral gray background (and I'm assuming the OP used the same background and exposure for all these).. First.. measuring color.. On a Mac, in the utilities folder is a tool called Digital Color Meter. It allows you to pick a sampling point and see the Red/Green/Blue (RGB) values. Some quick basics.. White is 255, 255, 255.. Black is 0,0,0.. Most photographers use an 18% grey card (Neutral Gray) to set White Balance (if doing manual WB before hand). That would result in a reading of 52, 52, 52 (about). The OP's gray background is measuring higher (using sRGB), but that isn't as critical as what the measurements all read for Red Green and Blue.
If we start with the DJI UV, the values are 148, 148, 140.. (I'm on a calibrated monitor - YMMV) .. The DJI ND 8 are 134, 135, 128.. Again all very close to one another in keeping the gray card gray. Now compare all the other backgrounds by each brand of filter. Some (like Polar Pro ND-8 Screw on and SRP ND 16/CP) are very lacking in Blue (compared to Red and Green) which is why they look so much "warmer".. The Neewer do an excellent job on color balance, as do the Anbee and DJI.. others too are close, but I'm looking for only single digit differences between RGB and numbers that and that are relative to the ND value of the filter. Since the OP was showing the effect of ND, the RGB values will reduce vs. the UV filter because the amount of "gray" is being reduced (assuming the OP used the same exposure - which it appears he did)... The "grays" should start to lean toward more black (again 0,0,0)...
Where my concern is (and the OP addresses it as well) is weight. Only the Anbee and Taco are the same as DJI - they also have very good RGB numbers.. Neewer doubles up on DJI weight, and the rest from there. I have a question out to DJI about how much "additional" weight are the motors supposed to be able to handle. Counterbalancing actually adds more TOTAL weight to the other motors.. So it's a balance (until I get an answer - hopefully) of color rendition, ND purity, and weight (and cost)..
If you shoot in RAW and use an editor, color correction can be done, as can exposure, etc. but it's always better to get it right in the camera vs. post production.. With certain products you can color correct a whole video, but again you always want to get it right before editing.